Power supplies with switched mode converters for transforming an input voltage to one or more different output voltages are known in the art. Primary current through a transformer from an input source voltage is controlled by a main power switch such as a high speed, high voltage, power switching transistor. The transistor is switched on and off to provide a changing rate of current through the transformer primary, i.e., cause an alternately collapsing field to couple and induce the flux into the transformer secondary and thus yield a given output voltage.
One of the problems encountered with switched mode converters is the need to protect the power switching transistor from reverse bias secondary breakdown, and thermal runaway. This problem is solved by using a turn-off current snubber to shape the load line.
One type of such snubber is an RC dissipative current snubber comprising a resistor, a diode and a capacitor. The disadvantage of the RC snubber is the excessive power dissipation within the resistor of the RC network. In the RC snubber, the energy stored on the capacitor during and following transistor turn-off must ultimately be dissipated within the network resistor. The wattage dissipated within the resistor will often exceed that dissipated in the switching transistor. Not only is efficiency reduced, but often an additional heat dissipator must be incorporated into the design to prevent overheating the neighboring components as well as the resistors themselves. For further background regarding RC dissipative current snubbers, reference is made to the background review sections of Whitcomb, et al., "Designing Non-Dissipative Current Snubbers For Switched Mode Converters", Proceedings of the Sixth National Solid-State Power Conversion Conference, May, 1979, and William J. Shaughnessy, "Modelling and Design of Non-Dissipative LC Snubber Networks", Proceedings of the Seventh National Solid-State Power Conversion Conference, March, 1980.
In order to improve system efficiency and avoid the wasteful RC dissipative current snubber, LC non-dissipative current snubbers have been developed, for example as shown in the above noted Whitcomb and Shaughnessy references. The LC snubber adds negligible power dissipation to the converter circuit thus easing the problem of heat dissipation and improving efficiency. The energy stored in the capacitor of the LC snubber is returned to the input source voltage line.
Another technique for conserving transformer leakage inductance energy is shown in Robert J. Boschert, "Flyback Converters: Solid-State Solution to Low-Cost Switching Power Supplies", Electronics, Dec. 21, 1978, pp. 100-104, FIG. 3. This design uses an RC snubber but also includes an auxiliary clamp winding having the same number of turns as the primary and connected back to the input source voltage through a rectifier diode. The diode clamps the leakage inductance spike on the primary switch to the input source voltage.
Another technique for conserving transformer leakage inductance energy is described in Toshio Fujimura et al., "Wide-Range and High Efficiency Switching Power Supply for Color TV Receivers", IEEE Transactions on Consumer Electronics, Vol. CE-24, No. 3, August, 1978 pp. 473-479. This technique uses interleaved transformer windings to reduce leakage inductance. This increases the cost of the transformer, and also makes it more difficult to insulate the primary from the secondary.
In addition to the above described need to improve overall system efficiency by conserving transformer leakage inductance energy, there is also a need for improved energy efficiency in the driving circuit for the pulse width modulator and the high voltage transistor. There is a need to provide power to the pulse width modulator and the transistor switch with minimum loss over a wide input voltage and frequency range.
The present invention affords a common solution to both of the above noted needs.